1. Field of the Invention
This invention relates to electric hammers having an air cushion hammering mechanism.
2. Description of the Related Art
Such hammers will normally have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit. The hammer is normally provided with an impact mechanism comprising a motor that drives a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of an air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer via a beatpiece located within the spindle.
Some hammers can be employed in combination impact and drilling mode in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein, will be caused to rotate at the same time as the bit is struck by the beat piece.
When the hammer is to be used the forward end of a tool or bit is pressed against a workpiece, which urges the tool or bit rearwardly within the hammer spindle. The tool or bit in turn urges the beatpiece rearwardly into its operating position in which the rearward end of the beatpiece is located within the reciprocating path of the ram. In the operating position the beatpiece receives repeated impacts from the ram. When the hammer is in use, the forward impact from the ram is transmitted through the beatpiece to the bit or tool and through the bit or tool to the workpiece. A reflected impact is reflected from the workpiece and is transmitted through the bit or tool to the beatpiece. This reflected, or reverse impact must be absorbed within the structure of the hammer in such a way that the reverse impacts do not over time destroy the hammer and so that the reverse impacts are not transmitted to the end user.
When the user takes the tool or bit of the hammer away from the workpiece, the next forward impact of the ram on the beatpiece urges the beatpiece forwardly into its idle mode position. The beatpiece can move forwardly and stay forwardly because the tool or bit is no longer urging it rearwardly, because the tool or bit also assumes a forward idle mode position. Because the beatpiece does not now offer much resistive force against the ram, the ram also moves into a forward idle mode position. In the idle mode position of the ram, the air cushion is vented and so any further reciprocation of the piston has no effect on the ram. This forward movement of the components on entry into idle mode generates the greatest impact forces on the structure of the hammer, in particular on the hammer spindle. This is because the forward impact force of these parts on entry into idle mode is not transferred to the workpiece, but has to be absorbed by structure of the hammer itself. Thus, the number of idle strikes, ie. the number of reciprocations of the ram, beatpiece and tool or bit, when the bit or tool is removed from the workpiece need to be minimised in order to minimise the number of high impact force idle strikes that have to be absorbed by the structure of the hammer. This is generally achieved by catching the ram and/or the beatpiece in their idle mode positions so that they cannot slip rearwardly to cause the ram to move into a position in which the air cushion is closed and the ram and thus the beatpiece begin to reciprocate again.
In order for the maximum impact to be transmitted from the ram to the tool or bit, via the beatpiece, the beatpiece must be co-axial with the spindle. Thus, high efficiency is achieved if the reciprocating movement of the beatpiece within the spindle is guided to ensure good axial alignment with the axis of the spindle.
Hammers are necessarily operated in very dusty and dirty environments. If dust gets into the spindle of the hammer it can cause abrasion between the reciprocating parts and, in particular, can cause seals between the ram and the spindle to become worn. Wearing of the seal around the ram will cause the air cushion to deteriorate, which will eventually lead to impacts occurring between the beatpiece and the ram which can seriously damage the hammer. Therefore, a further issue in the longevity of the working life of a hammer is its sealing against dust. The reciprocation of parts within the spindle can draw dust rearwardly inside the hammer spindle, where damage can be caused.
Attempts to solve these problems have been made and examples of the resulting hammer arrangements are known from U.S. Pat. No. 4,476,941 and DE196 21 610.
The arrangement in U.S. Pat. No. 4,476,941 has a complicated multi-part spindle arrangement with a first sleeve for guiding a rearward reduced diameter portion of the beatpiece, which sleeve extends from the inside to the outside of the spindle, between two spindle parts. The impact of the beatpiece on entry into idle mode is absorbed by a second sleeve, located forward of the first and within a different one of the spindle parts. The second sleeve also guides a forward increased diameter portion of the beatpiece. The arrangement in U.S. Pat. No. 4,476,941 has a problem with dust ingress, in particular during periods when a tool or bit is removed from the tool holder of the hammer, into the portion of the forward sleeve where the beatpiece is guided. This problem is exacerbated by the pumping nature of the increased diameter portion of the beatpiece which is guided within the second sleeve. The small amount of axial support for the first sleeve which is mounted between spindle parts, along with usual tolerance limitations for component parts could lead to a reduced accuracy of axial guiding of the beatpiece by the sleeves. The design in U.S. Pat. No. 4,476,941 results in a complicated multi-part spindle, beatpiece guiding and damping structure, with the associated assembly problems and cost implications.
The arrangement in DE196 21 610 overcomes some of the problems discussed above, but still has the disadvantage of a relatively complex three part spindle arrangement, having sleeves for beatpiece guiding mounted and guided in different spindle parts. Again the usual tolerance issues between spindle parts can reduce the accuracy with which the beatpiece is guided and complicates the sealing of dust from the inside of the spindle. Again the design in DE196 21 610 has a complicated multi-part spindle, beatpiece guiding and damping structure, with the associated assembly problems and cost implications.